1. Field of the Invention
The present invention relates to a toner for electrophotography, electrostatic recording, electrostatic printing, etc., a method for producing it, and a method of image formation with it.
2. Description of the Related Art
Toner for electrophotograpy (this will be hereinafter referred to simply as "toner") could be obtained by mixing, melting and kneading a binder resin, a colorant, and optionally a lubricant, an antistatic agent and other additives, cooling and solidifying the resulting mixture, grinding the solid, classifying the ground powder to prepare a toner matrix, followed by adding external additives such as a fluidizing agent, an antistatic agent, a cleaning promoter and the like to the toner matrix to thereby make them adhere to and fix on the surfaces of the toner matrix particles, and finally sieving the thus-processed toner particles to thereby remove coarse particles having been grown too large in the step of adding thereto the external additives.
In electrophotography with toner for image formation, the electrostatic latent image formed on a latent image carrier is developed with toner, then the toner image is once transferred onto an intermediate image receptor, and thereafter re-transferred onto an image-receiving object such as plain paper, plastic film or the like, and finally the toner image on the image-receiving object is fixed thereon under heat and/or pressure in a fixation unit generally equipped with a thermal fixing roll or belt system.
Therefore, the toner particles to be used in image formation through electrophotography have the capability of plastic deformation by heat and/or pressure. With the recent needs for power-saving and instant-on duplicators for rapid duplication, toner capable of melting at lower temperatures than usual is desired. However, toner that satisfies the requirement is often problematic in that it will fuse in the apparatus where it is produced.
Realizing high-quality images is especially desired these days, for which fine toner particles having a reduced particle size and having a narrowed particle size distribution are necessary. However, such fine toner particles will fuse more easily in the apparatus where they are produced. This is because re-structuring the apparatus for producing the intended fine toner particles will inevitably bring about the result that the particles produced therein fuse more easily. Concretely, for example, in the step of classifying toner particles, centrifugal classifiers heretofore used will have to be so modified that they enable high-revolution classification with an increased air flow. For their capabilities, however, the modified units for such high-revolution classification with an increased air flow being introduced thereinto are limited in producing the intended fine toner particles having a reduced particle size and having a narrowed particle size distribution, and will augment the fusion of the toner particles around rotors, etc.
In place of centrifugal classifiers, inertial classifiers where particles are classified by their inertial force are available on the market these days and are being used widely in the art. In the inertial classifier, particles having a large particle size are blown far away by their inertial force, while the others having a small particle size flow along a curved block, a Coanda block, owing to the specific capabilities of the classifier. Increasing the number of the edges for classification in the inertial classifier of that type enables multi-division classification. Even for small-sized particles, the inertial classifier could well classify them merely by moving the position of the edges without sacrificing its classification efficiency. Using the inertial classifier has made is possible to produce even fine toner having a reduced particle size and having a narrowed grain size distribution. In addition, only one inertial classifier enables multi-stage classification, for which, however, one centrifugal classifier must be repeatedly driven plural times. Having the advantages of reduced equipment cost and space with no reduction in the classification efficiency even for small-sized particles, inertial classifiers are being much used these days in the art.
However, inertial classifiers are still confronted with some problems. Concretely, in an inertial classifier, many toner particles fuse around the edges that define the classification point and around the ejector, and the fused toner particles will change the classification point. In that case, it becomes difficult to stably produce toner particles having the same quality (that is, having the same particle size distribution). Depending on the degree of toner fusion, the production condition must be re-settled, or the fused toner particles must be removed. This increases the cost for apparatus inspection, and greatly lowers the apparatus performance, as the apparatus must be dissembled and cleaned.
Also in the step of grinding toner raw material, the apparatus to be used must be modified so as to increase its grinding efficiency for the recent fine toner particles having a reduced particle size and having a narrowed particle size distribution. Concretely, in a jet-mill grinder, the air flow rate is increased; and for other mechanical grinders, the rotor revolution is increased. In particular, for grinding into fine toner particles having a narrowed particle size distribution, used is a grinding system equipped with a built-in pre-classifier for removing coarse particles or with an external closed circuit. In the grinding system of that type, however, toner particles fuse around the rotor in the built-in pre-classifier, and the fused particles will vary the classification point in the pre-classifiers. This is problematic as the toner particles ground could not have the intended particle size.
For increasing the grinding efficiency in producing fine toner particles having a reduced particle size, a method of varying the constituent monomers for the binder resin to be in toner, or a method of lowering the softening point or the glass transition point of the binder resin may be employed. These methods are effective for improving the grindability of toner particles, but are still problematic in that many toner particles fuse inside the grinder and the classifier used and even in the pipe line that connects the units, and the fused particles will soon come to have negative influences on the production condition within a short period of time. Moreover, it is known that the toner produced in the process where many particles fuse inside the production units contains an increased amount of coarse particles (having a large particle size of from 16 to 45 .mu.m or so). The coarse toner particles will form image defects of grids and white spots on duplications. In addition, they will have not a few negative influences on the charging characteristics and the fixing characteristics of toner. Even though fine toner particles could get improved grindability in some degree according to the methods as above, they will much sacrifice their intrinsic properties for it.
Another method that may be employable for the intended purpose will be to add to toner a material capable of improving the grindability of toner material. For example, in Japanese Patent Laid-Open No. H04-257868, proposed is a technique of using an aromatic petroleum resin for improving both the grindability and the fixing capability of toner; and in Japanese Patent Laid-Open No. H07-278658, proposed is a technique of using a hydrogenated petroleum resin for improving both the grindability and the thermal stability of toner. The techniques proposed could improve the grindability of toner, but are still problematic in that they worsen the charging capability of toner.
For producing the intended, fine toner particles that can melt even at low temperatures and have a reduced particle size and a narrowed particle size distribution, the problem to be solved is that particles fuse inside the production equipment and many coarse particles are formed therein. To solve the problem, another method has been tried. This is to coat the inner surface of the body unit of an apparatus for toner production as well as that of the pipe line around the apparatus and that of the peripheral units such as cyclones with a carbon-dispersed fluororesin. However, in the apparatus and through the pipe line around it and even in cyclones, toner particles all the time colloid/rub against their walls along with air streams running therethrough at a high speed of from 15 to 100 m/sec and even at a higher speed of from 100 to 200 m/sec. In that condition, even though the inner walls of the apparatus body and its peripheral lines and units are coated with such a fluororesin, the resin coating will be immediately peeled off as the resin is not tough and has poor abrasion resistance. Therefore, the resin coating could not be put into practical use. What is more, since the fluororesin is an insulating material, using it in toner production equipment is not practicable from the viewpoint of preventing accidental explosions.
For effectively overcoming the problem of toner fusion in production equipment, at present, no one knows a method of modifying the equipment for toner production, modifying the materials for the units of the equipment or modifying the toner materials.
With the recent tendency in the art toward full-color, high-quality duplications, it is desired that colorant could be dispersed in toner in the form of primary grains as much as possible, and hardly-dispersible, fine-grain colorants having a reduced grain size and having an increased surface area are being used in toner. At present, however, such fine colorant grains could not be dispersed in toner in the form of primary grains, even when a high-shear blender capable of realizing relatively high-degree dispersion is used for blending them. If colorant grains are poorly or locally dispersed in the binder resin in toner, or even though they are uniformly dispersed therein but it the colorant dispersion units are large, the toner containing them will be ground in the next grinding step after the step of melt-kneading the colorant grains in toner, while the colorant grains still exist locally in the toner or while the toner still contains large dispersion units of colorants. If so, the compositional constitution of the toner produced will be uneven. In particular, when the colorants are dispersed extremely poorly in the binder resin, the toner produced will partly contain segments of a single component only or particles having an uneven compositional constitution. As a result, the charge spectrum of the toner will be broadened, and, as the case may be, the toner will be charged in reversed polarity. Moreover, since the toner material is ground into fine particles while the colorant grains are exposed outside the toner particles or while they are released from the toner particles, the powdery fluidity of the resulting toner particles will be greatly lowered, or, as the case may be, the exposed or released colorant grains will cause filming on photosensitive materials.
For example, for full-color colorants, if they are poorly dispersed in toner, their transparency will be lowered, and the underlying colorant could not exhibit its color. As a result, the images formed could not exhibit the intended color. For monochromatic colorants, their color reproduction range will be narrowed, their saturation will be lowered, their coloring capability fill be lowered, the density of the images formed will be lowered, and the granularity of toner containing them will be lowered. Moreover, the charging difference between colors of yellow, magenta and cyan will increase, and the binder strength will be lowered. As a result, the poorly dispersed colorant grains will have some negative influences on the fixing capability of toner containing them, and, still another problem with them is that the grain size distribution in the toner particles having been ground in the grinding step is broadened. What is more, since the charge spectrum of toner is broadened, the background in duplications is often fogged, and the apparatus used will be readily soiled with toner whereby its running life will be shortened.
On the other hand, for particles for two-component black toner, the toner could not be charged to a satisfactory degree. In addition, the charge spectrum of the toner will be extremely broad, and the toner could not be admixed to a desired degree. As a result, the images formed will be fogged, and the apparatus used will be soiled.
For magnetic colorant powders, they will aggregate to give local segments of powder aggregates, and will case various troubles. For example, the segments will release free magnetic powders from them, and the free magnetic powders will scratch the surface of photoreceptors in duplicators, or will deposit thereon while being carried by the developer carrier. As a result, the density of the color images formed will be become gradually uneven. In order that these colorant materials are dispersed in toner in the form of primary grains as much as possible, for example, they may be previously flashed before use, or they are first formed into a master batch before being mixed in toner. As the case may be, a dispersant may be added to toner along with the colorant materials. However, even though such a dispersion promoter is used, it is not still effective for making the colorant materials well dispersed in toner in the form of primary grains. The method of flashing the colorant materials, and the method of preparing a master batch of colorants followed by mixing it in toner in plural stages are both problematic in that they increase the raw material costs and the production costs.
Various dispersion promoters have heretofore been proposed, but all of them are not satisfactory as they have some negative influences on the charge spectrum of toner or as they often detract from the coloring characteristics of toner for full-color duplications. For example, Japanese Patent Laid-Open No. H04-186370 discloses a phthalocyanine derivative that serves as a dispersion promoter. When the derivative is added to toner, the colorants could be effectively dispersed in toner in some degree, but their coloring capabilities are lowered. What is more, the color reproduction range of the colorants is narrowed, and the saturation thereof is lowered. When the derivative is added to two-component toner that contains carbon black, or to one-component toner that contains magnetic powder, it noticeably broadens the charge spectrum of the toner. As a result, the background in duplications is much fogged, and the apparatus used is much soiled.
At present, no one could obtain easily-meltable, fine-particle toner having a reduced particle size and having a narrowed particle size distribution, of which the advantages are that colorants can be uniformly dispersed therein to a great extent, that the toner particles do not fuse in the equipment where they are produced, and that the amount of coarse particles having a large grain size in the toner is small. Under the current situation, it is still difficult to produce fine toner particles having a reduced particle size in an efficient manner not having any negative influences on the toner characteristics, especially on the charging characteristics and the coloring characteristics of the toner produced.